It's funny how while software changes so fast, and many hardware components evolve at ridiculously fast paces (processors, memory, hard drives), the keyboard has remained largely unchanged over the years - until recently, that is. Even Lenovo has now buckled under the pressure, switching to a chiclet-style keyboard for ThinkPads - while also removing the SysReq key.

I haven't used the Break key since my DOS days (usually in conjunction with the Ctrl key). It was used to preemptively terminate running applications. It was implemented as a system call (or interrupt) if I remember correctly, but it could be blocked/ignored by an application.